This invention relates to aluminium-base alloys, and more particularly to aluminium-base alloys capable of being formed or shaped into objects by superplastic deformation.
It is known that certain alloys under certain conditions can undergo very large amounts of deformation without failure, the phenomenon being known as superplasticity and characterised by a high strain rate sensitivity index in the material as a result of which the normal tendency of a stretched specimen to undergo preferential local deformation ("necking") is suppressed. Such large deformations are moreover possible at relatively low stresses so that the forming or shaping of superplastic alloys can be performed more simply and cheaply than is possible with even highly ductile materials which do not exhibit the phenomenon. As a convenient numerical criterion of the presence of superplasticity, it may be taken that a superplastic material will show a strain rate sensitivity ("m"-value) of at least 0.3 and a uniaxial tensile elongation at temperature of at least 200%, m-value being defined by the relationship .sigma. = .eta..epsilon..sup.n where .sigma. represents flow stress, .eta. a constant, .epsilon. strain rate and n strain rate sensitivity index.
No known aluminium-base alloy can be superplastically deformed other than the Al-Cu eutectic composition which contains 33% copper and has neither the low density nor the good corrosion resistance characteristic of aluminium alloys.
In the Complete Specification of copending British patent application No. 33922/71 (U.S. pat. Ser. No. 273,639) now U.S. Pat. No. 3,876,474 aluminium base alloys are described which can be superplastically deformed. These include non-heat treatable alloys containing at least 5% magnesium or at least 1% zinc, together with at least one of the elements Zr, Nb, Ta and Ni in a total amount of 0.3% to 0.8% substantially all of which is present in solid solution.
It was not found possible, by inclusion of even considerable amounts of zirconium in the desired form, to induce superplastic behaviour in pure aluminium or the Al-11/4% Mn alloys or in Al-Mg alloys containing a few percent only of magnesium, although these are the cheapest and most widely used types of aluminium alloy for production of formed components.